This single-arm phase 2 trial (ChiCTR2100046715) examined previously untreated patients with advanced esophageal squamous cell carcinoma (ESCC) who received four cycles of paclitaxel with carboplatin every 3 weeks. Toripalimab was infused intravenously every 3 weeks for 12 months, or until disease progression or intolerable toxicity. Radiotherapy that encompassed the primary lesions and metastases commenced in the third cycle. The median progression-free survival time was 9.8 months (95% confidence interval [CI]: 6.8-not estimable) in the intent-to-treat population, failing to meet the pre-specified primary endpoints. Secondary endpoints included an objective response rate of 45.5%, a disease control rate of 57.6%, and a median duration of response of 11.5 months (interquartile range, 6.4-15.0). The 1-year progression-free survival and overall survival rates were 41.9% (95% CI: 27.7-63.5) and 69.7% (95% CI: 55.7-87.3), respectively. Lymphopenia was the most frequent grade >= 3 adverse event (82%), and an esophageal fistula developed in three patients (9.1%). No treatment-related deaths occurred. In prespecified exploratory biomarker analysis, higher densities of CD8 + T cells, CD11c+ dendritic cells, and CD68+ macrophages correlated with improved tumor response and prognosis. Radiotherapy supplementation to first-line chemo-immunotherapy for treatment-naive advanced ESCC demonstrated some antitumor activity and manageable safety profiles, warranting further randomized controlled trials. Chemo-immunotherapy regimens are now recommended as first-line treatment for patients with advanced esophageal squamous cell carcinoma (ESCC), however survival outcomes remain unsatisfactory. Here the authors report the results of a phase 2 trial of toripalimab (anti-PD1) plus chemotherapy and radiotherapy in patients with treatment-na & iuml;ve advanced ESCC.